🚀 go-pugleaf

In article <C0rp00.9q2@news.cso.uiuc.edu> dnash@uxa.cso.uiuc.edu (Dave Nash) writes:

>In article <1993Jan12.224443.1547@linus.mitre.org> chaloux@mandolin.mitre.org (Dave Chaloux) writes:

>>An images of a star in focus consists of an Airy Disk and diffraction rings
>>around that disk. An unobstructed telescope like a refractor can, if it is
>>perfectly made put about 84% of the light into the central disk. Any obstruction
>>(or under or over correction of the optics) puts more of the light into the
>>diffraction rings.

>...

>>The answer isn't this simple however. Obstructed reflectors of larger diameter
>>can be gotten for much less money than equivalent refractors. If the reflector
>>is well made, the larger diameter of the mirror can more than make up for the
>>loss of the light due to the obstruction. Doug McDonald has said on numerous
>>occasions on the Net that given perfect optics, a Reflector of size a and
>>with an obstruction of size b will perform as well as a refractor of size
>>a - b/2. 

Caveat:  I'm saying that this applies to fine details on planetary disks.
Not to looking for the companion of Sirius or Deimos or Phobos.


>This reminds me of a (possibly naive) question I have on this topic:

>Some people use this argument (obstructions causing more light in the
>diffraction pattern to be in the outer rings) to argue that small refractors
>have an intrinsic advantage over significantly larger reflectors.
>Is it possible for a larger, obstructed scope to be largely immune to
>this effect (more light in the rings) on the diffraction pattern, when
>compared to a smaller, unobstructed telescope, like a refractor?
>True, obstructed scopes dump more light into the rings, but the diffraction
>pattern of a large scope is so much smaller that this would seem to have no 
>real effect -- especially if the larger scope is sufficiently larger that
>the diameter of its first diffraction ring is less than that of the Airy
>disk alone of the smaller scope.  Is this reasonable?


>I'm not an optics jock, so i'll yield to those who are.

That's about right. I've got the calculations right here (i.e. 50 feet from
the present poster's office) in Mathcad.

It seems that a twice-size SCT is better at all distances away from the
central star than a perfect refractor. Come look. Actually it is far better
than that.

Again, we assume perfect optics.

Doug McDonald